Improvement in wood-screws



Wood-Screw.

Patented Dec I 8 Zim? 2.

e T w J UNITED STATES PATENT OFFICE'.

HAY WARD A. HARVEY, OF ORANGE, NEW JERSEY.

IMPROVEMENT IN WOOD-SCREWS.

Specification forming part of Letters Patent N0. 197,933, dated December11, 1877; application filed April 20, 1877.

To all lwhom t may concern:

Be it known that I, HAYWARD A. HARVEY, of Orange, New Jersey, haveinvented a certain Improvement in Wood-Screws, of which the following isa specification:

My improvements relate to the formations of threads upon wood-screws.

The chief feature of my invention consists in forming such threads witha differential pitch and a variable depth, the pitch and the depthhaving a constant relation to each other, due to the fact that the coreis of the same length between any two threads, whatever may be thepitch. Owing to this peculiarity I am enabled to cut a thread ofdifferential pitch with a tool having a single fixed lead-in otherwords, without varying the character of the lead given to the tool inmaking the series of cuts -necessary to finish the thread to the vproperdepth.

I form my thread with a gradually-increasing pitch from its point ofcommencement upon the shank of the blank, which, for con venience, Icall the heel7 of the thread. I may gradually increase the pitch fromthe heel to the point of the screw, or increase it for only a portion ofthe distance and cut the remainder of the thread upon a uniform pitch.The latter mode, especially, I contemplate adopting in the case of verylong screws.

rlhe accompanying drawings represent central longitudinal sections ofscrews exhibiting my invention in the two forms I have mentioned.

Figure 1 shows the diiferential thread commencing at the heel on theline a@ .r and ter- 'minating at the line y y, from which line thethread has the ordinary uniform pitch to the point of the screw. Fig. 2shows the differential thread having the pitch gradually increasing fromthe heel to the point. Fig. 3 is l a drawing showing the contour of thethreads and exhibiting the relation of the depth to the differentialpitch.

It will be seen that all the threads are nished to a sharp edge by thesame tool. In cutting the thread the lead of the tool is graduallyincreased in speed, and at the same time the tool is fed inward towardthe axis of the blank.

I determine the character of the motion required to be given to the toolby dividing the which the differential thread makes. For example, inFig. l the differential thread makes only four turns around the core. Tofind the motion for the tool in this case, I lay out a single thread, asin Fig. 3, and then draw, at equal distances, four lines, a b c di,parallel with the bottom line c, which represents the length of thecore. I make the. lines a b c d of the same length as the bottom line e,measuring from their respective points of intersection with 011e of theinclined sides, A, of the thread. Three inclined lines, b, c', and d',are then drawn parallel to the opposite inclined side e of the thread.The distance from the line b to the line a. indicates the depth attainedby the thread at one revolution. The distance of the line b from thepoint a indicates the gain of the lead while the blank ismaking a singlerevolution. The distances between the lines a, b, c, d, and e,respectively, indicate the gain in the depth of thread at each of itsrevolutions, and the disstances between the point a on the line a andthe respective points where thelines bf, c', d', and c intersect theline a indicate the increase in pitch at each revolution, or, in otherwords, indicate the amount of increased lead required to be given to thetool for each of' the four successive turns of the blank.

It will be seen that the lines representing the core in Figs. l and 2are inclined to the axis of the screw.

In measuring the depth of the thread preparatory to laying off the pitchof the diiferential threads, as I have described, I measure on thedotted lines Z, which indicate the averageV depth of the deepest thread.In Fig. 2 there are twelve turns of the differential thread, and Itherefore divide the greatest depth into twelve equal parts.

It will, of course, be seen that the principle of laying out thedifferential threads as I have described, for the purpose of measuringthe pitch, is capable of application for any variety of pitch required.

I claim as my invention- A screw having `a thread which, for any portionof its length, has a differential pitch and a correspondingly variabledepth', substantially as described.

\ H. A. HARVEY.

Witnesses Ask FARE, Enwn. PAYsoN. A

